Method of electrochemical machining

ABSTRACT

The invention relates to the metalworking field, particularly to electrochemical sizing machining, and can be used for manufacturing of machine workpieces having an intricate profile and shaping furniture from chromium-containing steels and alloys operating in aggressive environment under excessive friction. 
     Technical effect: improving machining accuracy by forming a lustrous layer on the machined surface and reduction of concentration of hexavalent toxic chromium ions in a waste electrolyte solution. 
     Summary of invention: in the initial step, the unipolar electrochemical machining by operating pulses of normal polarity is carried out forming a layer enriched with chromium ions in the electrolyte area adjacent to the workpiece surface, then, upon achievement of the predetermined machining depth, shape and size of the workpiece, the operational current pulses of normal polarity and the machining electrode feeding are turned off and the residual polarization voltage value at the interelectrode gap is measured using the test high-frequency pulses of normal polarity, then low voltage pulses of opposite polarity synchronized with the phase of maximal approximation of the electrodes to each other are turned on and chromium cathode deposition onto the machined workpiece surface is carried out by means of alternating the pulses of opposite polarity with test high-frequency pulses of normal polarity and controlling the chromium deposition by increment of residual polarization value relative to its value after operational pulses of normal polarity.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefits from the Russian Application RU2011101550 filed on Jan. 17, 2011. The content of this application ishereby incorporated by reference and in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to the metalworking field, particularly toelectrochemical sizing machining, and can be useful in manufacturingintricately profiled surfaces of machine components and shapingfurniture from chromium-containing steels and alloys operating inaggressive environment under excessive friction. More particularly, theinvention is directed to form a protective chromium layer on a machinedsurface of a workpiece within a single electrochemical machining step,wherein the protective layer will have low roughness and thus, lustrousfinish and provide high corrosion resistance and low frictioncoefficient as well as reducing the concentration of toxic hexavalentchromium ions in a waste electrolyte solution.

A method of pulse electrochemical machining (ECM) with DC voltage supplyto electrodes within a pause between operational pulses is known, inwhich the DC voltage value is set lower than an electrolytedecomposition potential [Russian Patent Specification No. 506484, B 23 H3/00, Bulletin of Inventions, Iss. 10, 1976].

The drawback of the known method resides in the fact that supplying tointerelectrode gap DC voltage lower than an electrolyte decompositionpotential, first, does not provide improvement of precision sinceconstant charge of double electric layer on the metal-electrolyteinterface decreases localization of anodic dissolution process and, andsecond, does not provide improvement of the surface quality (i.e.,reduction of roughness and increase of corrosion resistance) since theconditions for forming high-quality chromium layer on the surface of aworkpiece being processed are not defined. Another shortcoming is theabsence of information about a point of time and conditions for thechromium layer formation as well as methods to control the formation ofsaid layer.

A method of electrochemical machining chromium-containing steels inalkali metal nitrate-based electrolytes is known, in which the amplitudeof a positive half-wave of the current (of normal polarity) is more thanthe negative one [Electrokhimicheskaya obrabotka metallov. Moroz, I. I.et al. Moscow, Machinostroenie Pub., 1969, pp. 64-65, 130].

The drawback of the known method resides in the fact that conditions forforming on a machined surface a chromium layer with high lustre are notdefined. The conditions for reducing a concentration of toxic hexavalentchromium ions in a waste electrolyte solution during machiningchromium-containing steels and alloys are not defined either. Moreover,continuous alternation of normal and opposite half-waves will result inanodic dissolution of the workpiece surface; therefore, the chromiumlayer will be dissolved even if it is formed during previous oppositehalf-wave. The information about a point of time and conditions forchromium layer formation as well as methods to control formation of thelayer is not presented.

A method of electrochemical machining is known [U.S. Pat. No. 4,213,834,B23H3/02; B23H3/00; Jul. 22, 1980] in which in order to carry out aprocess at small interelectrode gaps, a signal representative of thevoltage pulse distortion (when using a current source) is used. Moreparticularly, a signal proportional to a maximal value of the secondderivative with respect to pulse voltage is used.

The method allows carrying out the machining at irreducibleinterelectrode gaps along with providing high copying precision.However, the method does not disclose conditions for forming on amachined surface a lustrous chromium layer as well as reducing aconcentration of toxic hexavalent chromium ions in a waste electrolytesolution during machining chromium-containing steels and alloys. A pointof time and conditions for the chromium layer formation as well asmethods to control the process of formation are not defined, either.

A method of electrochemical sizing machining is also known [RussianPatent No. 2038928, Oct. 10, 1990] in which the machining is carried outusing a pulse power supply with steeply-falling current-voltagecharacteristics and oscillation of one of the electrodes, wherein aninstant present value of voltage pulses is being controlled by selectingvoltage spikes at sites of electrodes approximation to each other andmoving apart and therewith increasing a machining electrode feedingspeed until the third local voltage extreme in the pulse midpoint isformed, and maintaining this speed while keeping the following ratio

0<(Ul.e.−Umin)/Umin≦0.2,

where Ul.e.>Umin is the voltage amplitude of the third local extreme;

Umin is the minimal voltage value.

The method allows performing the machining at irreducible interelectrodegaps along with providing high copying precision during copy-piercingprocess and obtaining high electrolyte pressure in the interelectrodegap. However, formation of a voltage local extreme in the voltage pulsemidpoint using machining electrodes made of 0.2-0.3 mm thick plate(foil) is impossible. It is explained by low rigidity of such electrodesthat does not allow increasing the electrolyte pressure in aninterelectrode gap and obtaining a signal for controlling the machiningprocess, the signal being in the form of the voltage third local extremein the pulse midpoint. However, the method does not disclose conditionsfor forming on a machined surface a lustrous chromium layer as well asreducing a concentration of toxic hexavalent chromium ions in a wasteelectrolyte solution during machining chromium-containing steels andalloys. A point of time and conditions for the chromium layer formationas well as methods to control the process of formation are not defined,either.

A method of ECM of an electrically conductive workpiece in anelectrolyte by supplying bipolar pulses between the workpiece and anelectrically conductive electrode is known, wherein one or more currentpulses of normal polarity are being alternated with voltage pulses ofopposite polarity [U.S. Pat. No. 5,833,835, B23H3/02; B23H3/00; Nov. 10,1998].

The method is the closest one to the inventive method, and we accept itas the closest prior art.

The drawback of the known method resides in the fact that although themethod allows conducting the machining at irreducible interelectrodegaps providing high copying precision during copy-piercing process toobtain high electrolyte pressure in the interelectrode gap, it isimpossible to form a voltage local extreme in a pulse midpoint byincreasing feeding speed when using machining electrodes made of 0.2-0.3mm thick plate (foil). As was indicated above, it is explained by thelow rigidity of such electrodes that does not allow increasingelectrolyte pressure in the interelectrode gap and obtaining a signalfor controlling the machining process, the signal being in the form ofthe voltage third local extreme in the pulse midpoint.

Moreover, while implementing this method, a pulse of opposite polarityis supplied at relatively large interelectrode gaps when an oscillatingelectrode is backed out from the surface of a workpiece to a largedistance thereby decreasing the efficiency of opposite polarity pulsesfor producing a lustrous surface by deposition of chromium from theelectrolyte. Thus, at large gaps the fluid resistance of IEG isdecreased, the electrolyte rate is increased and the flow becomesturbulent that hinders relatively slow cathode deposition processes.Changing the ambient pressure at the IEG entry with the frequency of10-100 Hz is difficult enough from the viewpoint of technology. And thesupply of pulses of opposite polarity just after the pulses of normalpolarity when the electrodes are moved apart results in reduction ofelectrolyte pressure in the interelectrode gap at the instance andbeginning of intense gas-filling of a medium between the electrodes dueto the boiling of overheated electrolyte and increasing the volume of agas phase accumulated in the electrolyte during the positive half-waveperiod. The properties of such vapor-gas electrolyte mixture becomesubstantially heterogeneous throughout the surface being machined. Uponthat, the electrolyte conductivity is dramatically decreased resultingin general increase of the process power consumption.

Thus, none of the known methods of ECM when applied to machining ofworkpieces made of chromium-containing steels does not provideachievement of high copying precision and formation of a lustrouschromium layer on the machined surface as well as reducing aconcentration of toxic hexavalent chromium ions in a waste electrolytesolution within a single technological operation.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to improve the quality ofmachining by forming a lustrous layer on a machined surface, and reducea concentration of toxic hexavalent chromium ions in a waste electrolytesolution by means of machining at small interelectrode gaps using pulsesof normal polarity at high current density as to form a polished surfacefollowed by depositing chromium onto the machined surface using pulsesof opposite polarity as to provide high lustre.

The object is attained by providing, in one aspect of the invention, amethod of electrochemical machining of chromium-containing steels andalloys in electrolytes comprising aqueous solutions of alkali metalsnitrates, wherein oscillation is being imparted to a machining electrodewhile bipolar current pulses synchronized with machining electrodeoscillations are supplied to an interelectrode gap, and the machining iscarried out at minimal gaps with controlled feeding speed. In accordancewith the invention, in the initial step, unipolar electrochemicalmachining by operating pulses of normal polarity is carried out to forma layer enriched with chromium ions in the electrolyte area adjacent toa surface of the workpiece. Then, upon achievement of a predeterminedmachined depth, shape and size of the workpiece, operating currentpulses of normal polarity and the machining electrode feeding are turnedoff and a value of residual polarization voltage at the interelectrodegap is measured using test high-frequency pulses of normal polarity.Then low voltage pulses of opposite polarity are turned on whilesynchronizing the pulses supply with a phase of maximal approximation ofthe electrodes to each other, and cathode deposition of chromium ontothe machined surface of the workpiece is carried out by means ofalternating the pulses of opposite polarity with test high-frequencycurrent pulses of normal polarity along with controlling a process ofchromium deposition by determining an increment of the residualpolarization value with respect to the corresponding value afteroperating pulses of normal polarity.

In addition, in accordance with one embodiment of the invention, theupper limit of amplitude and duration of pulses of opposite polarity isbounded proviso that the etching of machining electrode operationalsurface is absent while the lower limit of amplitude and duration ofpulses of opposite polarity is bounded proviso that continuous chromiumlayer is formed onto the machined workpiece surface.

In addition, in accordance with one embodiment of the invention, theduration of test voltage pulses of normal polarity is set within therange of 10-50 μs with frequency of 5-10 kHz while the amplitude—within6-8 V.

In addition, in accordance with one embodiment of the invention, thevalue of increment of residual polarization relative to its value afteroperational pulses of normal polarity is empirically set during first2-3 workpieces from the batch.

In addition, in accordance with one embodiment of the invention, atsupply of current pulses of normal polarity the electrolyte pressure atthe entrance of interelectrode gap is reduced to 50-150 kPa, andchromium deposition is carried out under the resulted electrolyte ratein the interelectrode gap.

In addition, in accordance with one embodiment of the invention, at themachining by operational pulses of normal polarity the size ofinterelectrode gap is reduced by gradual increase of machining electrodefeeding speed upon the first breakdown of the interelectrode gapthereafter the feeding speed is reduced by 3-10% relative to the speedat which the breakdown occurred, and the machining is continuedrepeating this action if desired.

In addition, in accordance with one embodiment of the invention, themachining by operational current pulses of normal polarity is carriedout in following modes: voltage on IEG is 5-15 V, electrolyte pressureat the IEG entrance is 50-500 kPa, electrolyte concentrations are 7-15%,and electrolyte temperatures are 18-40° C., ensuring current densitywithin 50-1000 A/cm².

In addition, in accordance with one embodiment of the invention, thepolarization voltage is measured at the end of the last test pulse inthe initial point of residual polarization decay curve, the duration oftest pulse group being selected proviso that the polarization voltageachieves a steady-state value.

In another aspect of the invention, an apparatus for electrochemicalmachining of chromium-containing steels and alloys in electrolytes basedon aqueous solutions of nitrate of alkali metals is provided, whereinthe apparatus comprises an oscillating machining electrode, a speedregulator for regulating the speed of feeding the instrument to maintainthe minimal interelectrode gap, and a current pulse generator forgenerating pulses of bipolar current synchronized with machiningelectrode oscillations for supplying to interelectrode gap,

wherein, the current pulse generator in the initial step of unipolarelectrochemical machining generates operating pulses of normal polarityto form a layer enriched with chromium ions in the electrolyte areaadjacent to the workpiece surface, the apparatus further comprising

a measurement unit for measuring a residual polarization voltage at theinterelectrode gap using test high-frequency pulses of normal polarity,in the state once the predetermined machining depth, shape and size ofthe workpiece are achieved and the operational current pulses of normalpolarity are switched off and the machining electrode feeding isstopped, and

wherein the current pulse generator generates low voltage pulses ofopposite polarity synchronized with the phase of maximal approximationof the electrodes with each other to enable chromium cathode depositiononto the machined workpiece surface by alternating pulses of oppositepolarity with test high-frequency pulses of normal polarity, and whereinthe chromium deposition is controlled by increment of residualpolarization value relative to its value after operational pulses ofnormal polarity.

In still another aspect of the invention, an article of manufactureobtained by a method according to the first aspect is provided, whereinthe article has a protective chromium layer on a machined surface,wherein the protective layer provides at least one of low roughness,lustrous finish, high corrosion resistance and low friction coefficient.

The inventive method of electrochemical machining of chromium-containingsteels and alloys allows improving the machining accuracy and producinglustrous finish as well as reducing the concentration of hexavalenttoxic chromium ions in a waste electrolyte solution.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Furthermore, the proposed invention is illustrated by non-limitingexamples of realization and accompanying figures confirming thepossibility of its realization on which:

FIG. 1 illustrates the process flow diagram in accordance with theinvention;

FIG. 2 illustrates oscillograms of voltage and current in IEG at thestep of shaping by pulses of normal polarity and at final step duringchromium deposition onto the machined surface in accordance with theinvention, where S is a track of oscillating machining electrode, mmdepending on time t, s, U is voltage of pulses of normal polarity, V,U_(A) is voltage of residual polarization after test pulses succeedingthe shaping step, V; U_(B) is voltage of residual polarizationdetermined by test high-frequency pulses after final step of chromiumdeposition, V; j is technological current density of pulses of normalpolarity, A/cm².

FIG. 3 a illustrates oscillograms of residual polarization voltage afterpulses of normal polarity determined by high-frequency pulses of normalpolarity (curve 1), and oscillograms of current of high-frequency pulses(curve 2) in IEG at the step of shaping by pulses of normal polarity,

FIG. 3 b illustrates oscillograms of residual polarization voltage afterpulses of opposite polarity determined by high-frequency pulses ofnormal polarity (curve 1), and oscillograms of current of high-frequencypulses (curve 2) in IEG in the final step during machining by pulses ofopposite polarity at chromium deposition onto the machined surface,

FIG. 4 illustrates the control structure of technological currentgenerator and source of current of opposite polarity of electrochemicalmachine in accordance with the invention, where: 3 is a controllabletechnological current source; 4 is a controllable source oftechnological current of opposite polarity; 5 is a controllable sourceof current (test pulses) of normal polarity; 6 is an electronic switchof the technological current source; 7 is an electronic switch of thesource of current of opposite polarity; 8 is an electronic switch of thesource of test pulses; 9 is an automatic control system of thetechnological process; 10 is a control block of the generator.

FIG. 5 illustrates an appearance of the machined workpiece surfaces withcorresponding profilograms after ECM using unipolar current pulses ofnormal polarity (A) and after ECM pulses of opposite polarity (B) inaccordance to the provided method;

FIG. 6 illustrates ratio of chromium to iron concentration in thesurface layer after ECM by unipolar pulses of normal polarity (A) and atECM by pulses of opposite polarity (B) in accordance to the providedmethod derived from the method of secondary ion mass spectroscopy.

DETAILED DESCRIPTION OF THE INVENTION

The invention is illustrated for better understanding by way ofnon-limiting example embodiments thereof, which are discussed in moredetail below

The process flow diagram of pulse bipolar electrochemical machining(ECM) by an oscillating EDM electrode at ET series machines in its mostgeneral form is illustrated in FIG. 1. As shown in FIG. 1, a machiningelectrode 1 performs periodic oscillations S(t) relative to the machinedsurface of the blank electrode 2 uniaxial to the feeding direction Vk.

At the beginning, near the phase of maximal approximation of theelectrodes to each other, an operational current pulse or group ofpulses of normal polarity and high density (within the range of 50-1000A/cm²) is supplied, then, upon the upon achievement the predeterminedmachining depth at final step of the process, the current pulses ofnormal polarity and the machine feeding are turned off, the totalresidual polarization voltage value U_(A) after supply of group of testlow voltage pulses is determined (FIG. 2), and low voltage pulses ofopposite polarity are turned on, the instant of the pulses of oppositepolarity supply being synchronized also with the phase of maximalapproximation of the electrodes, and chromium cathode deposition ontothe machined workpiece surface is carried out from the electrolyte atsmall interelectrode gaps. Hereupon, the test high-frequency pulses areagain turned on in order to determine residual polarization totalvoltage value after pulses of opposite polarization U_(B). Then the lowvoltage pulses of opposite polarity are turned on, the instant of thepulses of opposite polarity supply being synchronized also with thephase of maximal approximation of the electrodes to each other, andchromium cathode deposition onto the machined workpiece surface iscarried out from the electrolyte at small interelectrode gaps. Hereupon,the test high-frequency pulses are again turned on in order to determineresidual polarization total voltage value after pulses of oppositepolarization U_(B).

Changes in polarization values U_(A) and U_(B) are determined after thecurrent being turned off, thereby excluding the resistive component fromthe measuring voltage value and improving the reliability of measurementof difference between U_(A) and U_(B) that is defined as arepresentative parameter of enrichment of the machined surface bychromium (FIG. 3).

The test pulses ensure recharging of double electric layer capacity,establishment of the value of polarization that consists from anode andcathode potential. Hereupon, cathode potential is established morerapidly than the anode one, and its steady-state value at fixed currentdensity is stable while the anode potential value depends on propertiesof the machined surface and makes a major contribution to the incrementof residual polarization value U_(A) and U_(B). The size of group oftest pulses being selected proviso that the polarization voltageachieves a steady-state value.

Moreover, the test pulses alternated with the pulses of oppositepolarity can provide more favorable conditions for chromium depositiononto the machined surface since the electrolytic brightening ensuringlarge amount of nuclei and good adhesion with the support is consideredto be the best method of preparation of the surface for metal deposition[Povetkin, V. V. Structure of electrolytic coatings (inRussian)/Povetkin, V. V., Kovenskij, I. M. Moscow. Metallurgy Pub. 1989.136 P.].

The proposed method of electrochemical machining of chromium-containingsteels in electrolytes based on aqueous solution of alkali metalnitrates is carried out in electrolyte flow with superposition ofoscillations to one of the electrodes (FIG. 1). A source with steepcurrent-voltage characteristic (FIG. 4) which is periodically connectedto IEG by electronic switch of technological current source 6 near thephase of maximal approximation of electrodes to each other is used aspower supply 5 of normal polarity (Ip). Time of closed condition of theelectronic switch of technological current source 6 dictates theduration of current pulse (ti) of normal polarity (Ip).

The current pulses of opposite polarity (In) flow through IEG is ensuredby turning on the electronic switch of source of current of oppositepolarity 7 (see FIG. 4).

A source with steep current-voltage characteristic (FIG. 4) which isperiodically connected to IEG by electronic switch of test pulse source8 is used as the test pulse generator.

Increase of chromium amount onto the machined surface after bipolarelectrochemical machining is confirmed by results of determination ofsurface layer composition carried out using different methods.

Studies of surface of the machining electrode made of 40×13 steel werecarried out after unipolar ECM and bipolar ECM with an additional pulseof opposite polarity after operational current pulse when thesignificant changes in machined surface quality are taking place (FIG.5).

The experiments were carried out in 8% sodium nitrate NaNO₃ solution atcurrent density of operational pulse of ˜100 A/cm² and pulse duration of1.5 ms, current density and duration of pulse of opposite polarity of ˜5A/cm² and 2 ms, respectively. The instant of supply of the operationalpulses and pulses of opposite polarity was synchronized with the phaseof maximal approximation of EDM electrode to the machined surface. Theduration of test pulses was of 50 μs while the voltage amplitude wasselected to be not more than 8 V.

The results of assay of the surface layer by method of secondary ionmass spectroscopy showed increase of chromium relative to ironconcentration after bipolar ECM compared to unipolar ECM.

When using such type of surfaces in mating pairs, shaping tooling (dies,mandrels) etc. a friction coefficient is reduced and fatigue strength,wear-resisting properties and corrosion resistance are improved. Forexample, durability of a die from instrumental steel for production of“Torx” type hollows in steel screws increased more than twice comparedto the analogous die manufactured using conventional technology (bymeans of mechanical benchwork) and coated by titanium nitride. Thesimilar results are expected with dies for tablet formation (drugindustry).

It shall be appreciated that the prior art methods of unipolar machiningare usually connected with depletion of the surface layers ofchromium-containing steels in chromium. Advantageously, a method ofbipolar machining in accordance with the present invention provides theformation of chromium-containing layers on a wide range ofchromium-containing steels where the process is automaticallycontrolled.

In view of the high requirements for personnel protection andenvironmental protection from pollutants derived from electrochemicalmachining (ECM) of chromium-containing steels and alloys, study ofchanges in amount of bichromate ions in an electrolyte gains animportant role. It becomes even more important due to need in reducingthe wastes, from the economical standpoint (adoption of recirculationscheme) results in dramatic increase of electrolyte solution utilizationtime and, thereby, to increase of bichromate ions content in thesolution that will require the solution regeneration or replacementoperations.

However, the amount of bichromate ions in the solution could rather bereduced by means of their deposition onto the pre-machined surface athigh current density (e.g., larger that 100 A/cm²) by pulses of oppositepolarity in accordance with the provided method. Since the metal atomsin bichromate ions Cr₂O₇ ²⁻ have the maximal oxidation number they cannot oxidize on the positively charged anode, therefore they possess highstandard potential ((φ°Cr₂O₇ ²⁻/Cr³⁺=+1.33 V) and reduce on the machinedworkpiece surface upon to metal chromium. If the workpiece polarity willbe changed to negative and the corresponding conditions (e.g., smallinterelectrode gaps of 10-100 μm and voltage pulses of opposite polaritypreventing etching of the operational surface of machining electrode butsufficient for discharge of chromium ions on the machined surface) willpreliminary be provided, chromium deposition will occur in accordancewith the reaction:

Cr₂O₇ ²⁻+14H⁺+12e→2Cr+7H₂O.

EXAMPLE

A particular embodiment of the inventive method of electrochemicalmachining in accordance with the invention.

The inventive method of ECM by bipolar current pulses was carried out atET500 model electrochemical copy-piercing machine produced by “ECM” Ltd,Ufa, Russia, using 40×13 steel as the material of sample and machiningelectrode. The machining was carried out in aqueous 9.5% sodium nitratesolution upon the depth of 5 mm with area of 200 mm².

Before the machining step, an oscillating machining electrode 1 (FIG. 1)and machined blank 2 were approximated to each other upon the mutualcontact with the absence of technological voltage on them and movedapart to predetermined value of minimal interelectrode gap S_(t)=20 μm(FIG. 1).

Then, at the first machining step, the following mode of machining bypulses of normal polarity was set:

-   -   the frequency of rectangular current pulses and machining        electrode oscillations, Hz—49;    -   the duration of voltage pulse, ms—1.5;    -   the amplitude of machining electrode oscillations, mm—0.15;

The amplitude of rectangular voltage pulse at instant of minimaldistance between the electrodes, V—10.5;

The electrolyte pressure at the entrance of interelectrode gap, kPa—100;

The electrolyte temperature, ° C.—20.

The electrolyte feeding is direct through the central vent of machiningelectrode.

During penetration of machining electrode 1 (FIG. 1) into the blank 2upon the depth of 0.1-0.3 mm the feeding speed was of 0.1 mm/min. Then,with further penetration of the machining electrode 1 (FIG. 1) into theblank 2 the electrolyte pressure was gradually increased upon 350 kPa.During the machining by pulses of normal polarity the feeding speed wasgradually increased upon the first breakdown that corresponded to EDMelectrode feeding speed of 0.16 mm/min, then the feeding speed wasreduced by about 7% and the machining was continued upon thepredetermined depth.

Upon achievement of the predetermined depth of 5 mm the current pulsesof normal polarity and the machine feeding were turned off, and theresidual polarization total voltage was determined by turning on thetest high-frequency pulses with voltage amplitude of 7 V and pulseduration of 100 μs. Then the machining was turned on, in the initialstep low voltage rectangular pulses of opposite polarity with amplitudeof 3 V and duration 2 ms were set, the instant of supply of pulses ofopposite polarity being synchronized also with the phase of maximalapproximation of the electrodes to each other, and cathode deposition ofchromium from the electrolyte onto the machined surface at smallinterelectrode gaps was carried out with periodical measurements ofvoltage by test high-frequency pulses. Upon that, at the step ofchromium deposition, the pulses of opposite polarity were alternatedwith test high-frequency pulses of normal polarity ensuring the controlof chromium deposition upon the necessary increment of residualpolarization level relatively to its value after pulses of normalpolarity, the value of necessary increment being predetermined on 2-3workpieces within the batch.

When supplying pulses of opposite polarity, the electrolyte pressure wasreduced upon 100 kPa thereby forming laminar stream within theinterelectrode gap providing favorable conditions for deposition ofchromium from the electrolyte onto the machined surface. Hereupon, theamplitude and duration of pulses of opposite polarity were boundedproviso that the etching of machining electrode operational surface isabsent while the pulse amplitude and duration are sufficient fordischarge of chromium ions on the machined workpiece surface.

The analysis of machining results showed that upon the use of theprovided method a significant reduction of hexavalent chromium in thewaste electrolyte and producing a lustrous finish on the machinedsurface (Ra<0.15 μm) occurred, the EDM electrode copying error did notexceed 0.01 mm while the feeding speed value during the machining bypulses of normal polarity was of 0.15 mm/min.

1. A method of electrochemical machining of chromium-containing steelsand alloys in alkali metal nitrate aqueous solutions-based electrolytes,wherein a machining electrode is subjected to oscillations, and thepulses of bipolar current synchronized with machining electrodeoscillations, are supplied to an interelectrode gap, the method furthercomprising the step of controlling the speed of feeding the machiningelectrode to maintain minimal gap between the machining electrode andthe machined workpiece, wherein, in the initial step, the unipolarelectrochemical machining by operating current pulses of normal polarityis carried out to form a layer enriched with chromium ions in theelectrolyte area adjacent to the workpiece surface; then, uponachievement of the predetermined depth of machining, shape and size ofthe workpiece, the operating current pulses of normal polarity and themachining electrode feeding are turned off, and the residualpolarization voltage value in the interelectrode gap is measured usingtest high-frequency pulses of normal polarity; then low voltage pulsesof opposite polarity are turned on while synchronizing feeding thepulses of opposite polarity with the phase of maximal approximation ofthe electrodes to each other; and chromium cathode deposition onto themachined workpiece surface is carried out by alternating the pulses ofopposite polarity with the test high-frequency current pulses of normalpolarity while controlling the chromium deposition by increment of aresidual polarization value relative to the value obtained afteroperating pulses of normal polarity are applied.
 2. A method as claimedin claim 1, wherein the upper limit of amplitude and duration of thepulses of opposite polarity is bounded as to avoid etching of themachining electrode operating surface, while the lower limit ofamplitude and duration of pulses of opposite polarity is bounded as toprovide formation of a continuous chromium layer onto the machinedworkpiece surface.
 3. A method as claimed in claim 1, wherein theduration of test voltage pulses of normal polarity is set within therange of 10-50 μs with frequency of 5-10 kHz while the amplitude thereofis set within 6-8 V.
 4. A method as claimed in claim 1, wherein theincrement value of residual polarization relative to the value obtainedafter operational pulses of normal polarity are applied is setempirically using the first 2-3 workpieces from the batch.
 5. A methodas claimed in claim 1, wherein when feeding current pulses of oppositepolarity, the electrolyte pressure in the entrance into theinterelectrode gap is reduced to 50-150 kPa, and chromium deposition iscarried out with the resulted electrolyte rate in the interelectrodegap.
 6. A method as claimed in claim 1, wherein when the machining byoperating pulses of normal polarity is being performed, the size of theinterelectrode gap is reduced by increasing gradually the machiningelectrode feeding speed until the first breakdown of the interelectrodegap is occurred; then the feeding speed is reduced by 3-10% relative tothe speed at which the breakdown occurred, and the machining iscontinued while repeating this action, if necessary.
 7. A method asclaimed in claim 1, wherein the machining by operating current pulses ofnormal polarity is carried out in the following modes: voltage on IEG is5-15 V, electrolyte pressure at the IEG entrance is 50-500 kPa,electrolyte concentrations are 7-15%, and electrolyte temperatures are18-40° C. as to provide the current density within 50-1000 A/cm².
 8. Amethod as claimed in claim 1, wherein the polarization voltage ismeasured at the end of the last test pulse in the initial point ofresidual polarization decay curve, wherein the duration of test pulsegroup is selected such that a steady-state value of the polarizationvoltage is achieved.
 9. An apparatus for electrochemical machining ofchromium-containing steels and alloys in electrolytes based on aqueoussolutions of nitrate of alkali metals, wherein the apparatus comprisesan oscillating machining electrode, a speed regulator for regulating thespeed of feeding the instrument to maintain the minimal interelectrodegap, and a current pulse generator for generating pulses of bipolarcurrent synchronized with machining electrode oscillations for supplyingto interelectrode gap, wherein, the current pulse generator in theinitial step of unipolar electrochemical machining generates operatingpulses of normal polarity to form a layer enriched with chromium ions inthe electrolyte area adjacent to the workpiece surface, the apparatusfurther comprising a measurement unit for measuring a residualpolarization voltage at the interelectrode gap using test high-frequencypulses of normal polarity, in the state once the predetermined machiningdepth, shape and size of the workpiece are achieved and the operationalcurrent pulses of normal polarity are switched off and the machiningelectrode feeding is stopped, and wherein the current pulse generatorgenerates low voltage pulses of opposite polarity synchronized with thephase of maximal approximation of the electrodes with each other toenable chromium cathode deposition onto the machined workpiece surfaceby alternating pulses of opposite polarity with test high-frequencypulses of normal polarity, and wherein the chromium deposition iscontrolled by increment of residual polarization value relative to itsvalue after operational pulses of normal polarity.
 10. An article ofmanufacture obtained by a method of claim 1, having a protectivechromium layer on a machined surface, wherein the protective layerprovides at least one of low roughness, lustrous finish, high corrosionresistance and low friction coefficient.
 11. The article of manufactureof claim 10, wherein roughness of the machined surface is less than 0.15μm.